It may seem like a battery is a battery is a battery but there are some critical differences between batteries.
There are essentially two types of batteries you can choose from; AGM and Lead batteries. Many new BMW’s like E90 come with AGM batteries, these batteries use a technology called Absorbed glass mat. Without getting into the details essentially the benefit is that the battery does not emit any fumes like a lead battery does. There are other benefits like resistance to vibration, but those are irrelevant to normally driven cars. The price difference is almost double for an AGM over a lead battery.
It is worth quickly mentioning that AGM batteries are maintenance free, however for over a decade now lead batteries have also become maintenance free (they overfill the batteries at the factory which makes them maintenance free for their expected lifetime).
The reason you need to vent a lead battery is because 1. The gasses emitted will rust out metal very quickly in the trunk around the battery, but even more importantly 2. The gasses are explosive, and if not vented properly can cause an explosion (would take a very long time for this to potentially happen).
Most BMW’s will be the same in the way they are vented, I have showed step by step how to do the venting on an E30:
Most batteries do not come with a venting kit. The kit is fairly simple all you need is a L shaped plastic joint and some hose. Instead of paying $15-20 for the kit at the local parts store I decided to make my own kit that is of superior quality for far cheaper. I went to the BMW dealership and picked up the L shaped joint for $1.58 . The part number is: 61211377835
Then I went to the local hardware store (any hardware store will do) and I picked up some clear tubing, usually they sell it by the foot for around 15 cents/ft, but they were out of the cut-yourself rolls and I ended up having to buy the 20 ft pre cut roll. The size you will need is a tube that is 3/16″ inner diameter the outer diameter doesn’t really matter mine was 5/16″ OD.
This is what it will look like when you have re installed the battery and plugged the L shaped connector into the battery. There are two vent locations one on the positive side and one on the negative side, I chose to use the negative side:
Batteries are sized by “groups” the E30 uses a Group 48 battery regularly, what most people do not know is that the E30 tray will actually accept a Group 49 battery which is a bigger and more powerful battery. The Group 49 battery is what is seen above in an E30. There are two holes on the black tray for the hold down bracket, the one farther out is for a Group 49 battery. The fit is factory perfect.
Although there seems to be an enormous amount of companies selling batteries, it is worthwhile to know that the vast majority of them are rebranded batteries. The main three makers of batteries are: East Penn, Johnson Controls Inc, and Exide.
The batteries that originally came with your car and lasted 10 years can not be bought in North America they are made by a company in Europe called Varta. BMW dealerships for the longest time sold Douglas batteries which were of high quality, but have in recent years been switching to Exide which is generally agreed to be of lower quality.
East Penn and Johnson Controls are the two manufacturers I recommend. Napa sometimes carries East Penn batteries depending on which location you go to. Wal-mart sells a mix of all manufacturers but mainly Exide and Johnson Controls, the battery you see above is a Johnson Controls battery. Yes you can get a higher quality battery from wal-mart then the dealer for cheaper, not everyday you hear that.